1. Field of the Invention
This invention relates to frame detection circuitry for microfilm reader apparatus, and more particularly to such circuitry, in which the driving control of the microfilm is accomplished according to the detection of specific marks or frames on the microfilm.
2. Prior Art
According to the prior art attempts to detect the frames of a roll type microfilm, the distance between frame-sensing marks, for example such as blip-forming marks, bar codes or the like on microfilm F, which appear at positions corresponding to respective frames or a distance between the adjoining frames, as shown in FIG. 1, is detected by means of sensing marks M. More particularly, as shown in FIG. 2, light projector 1 including a light source, a condenser lens and the like is positioned in opposed relation to light receptor 2 including optical projector 3 and photoelectric converting element 4. Flashing light, transmitted through microfilm F due to the presence of frame-sensing marks or the frames themselves, is received by photoelectric converting element 4, and the output thereof is amplified by amplifier 5, as shown in FIG. 3, after which the outputs of amplifier 5 are counted (in the case of blip marks), or the width of an output signal is discriminated (in the case of a bar code), thereby detecting a desired frame. However, for achieving an accurate function of the aforenoted detecting means, it is mandatory that the quantity of light illuminated from projecting unit 1 onto a reading-out portion of microfilm F, which portion carries the aforenoted frame-sensing marks M or the like, be consistent or stable. In other words, where the quantity of light illuminated from light projector 1 is proper, an output of photoelectric converting element 4, which receives the light transmitted through a reading-out portion of microfilm F, exhibits pulse waveform Ao which is defined by a high amplitude corresponding to the light transmitted through mark portion M, and a low amplitude corresponding to the light transmitted through a portion other than the mark portion M, as shown by a solid line in FIG. 4. However, waveform Ao may be clipped at a given reference level H set relative to a "0" level, upon the shaping of the waveform. In this respect, where the quantity of light illuminated from light projector 1 is increased or decreased due to a voltage variation of an electric power source, or a variation in the quantity of light of a light source, for example due to deterioration during its service life, or due to stains and the like on a mirror included in projection element 3, the entire waveform is shifted up or down, as shown by a one-point chain line A.sub.2 and broken line A.sub.1 in FIG. 4, respectively, while reference level H remains constant, with the result that waveform A.sub.1 fails to detect mark M. In contrast thereto, in the case of a reader or a reader printer, in which the lens magnification may be varied, there is provided means, such as a variable resistor and the like, for adjusting reference level H to be used in the shaping of the aforementioned waveforms or the mark-detecting-sensitivity, commensurate with the quantity of light illuminated. However, such an adjusting operation requires an expenditure of much time and effort for changing the quantity of light being illuminated, with an accompanying detection error, if the operator commits a mistake in the adjustment, or in the worst case, the film itself is damaged. For those reasons, such an adjusting operation is not suitable for preventing a detection error accruing from a variation in the quantity of illuminating light.